1. Field of the Invention
The present invention relates to an optical scanning system and also to an image forming apparatus, employing the same, such as, for example, a copier, a facsimile device, a laser beam printer or the like for electrophotographically forming images.
2. Description of Related Art
In this kind of optical scanning system, a light source is modulated by an image signal to emit an image-wise laser beam representing an original image. The laser beam emitted from the light source is directed toward a rotating or swivelling deflection means and is reflected or deflected thereby to scan the photosensitive member in a primary scanning direction. By so doing, the photosensitive member is exposed to the imagewise laser beam, while moving in a secondary scanning direction perpendicular to the primary scanning direction. As a result of the primary and secondary scannings, an electrostatic latent image corresponding to the image signal is formed on the photosensitive member.
The formation of the electrostatic latent image must be conducted with high accuracy, and accomplishment of the image exposure of a sufficient resolution needs a large number of refractive forces in an image-forming optical unit and requires a light-flux to be shaped by the external configuration of a lens in the image-forming optical unit, to thereby control the configuration of a beam converging toward a plane to be scanned. These methods, however, have some inherent performance limitations. In view of this, the conventional optical scanning systems generally make use of different refractive forces in the primary and secondary scanning directions to compensate for shortage of the refractive forces, or employ a light-flux shaper corresponding to the external size of the lens in the image-forming optical unit to compensate for shortage of the force required to control the convergent beam configuration.
To overcome such problems, an optical scanning system employing an odd-shaped or axially asymmetric lens in the image-forming optical unit or employing a light-flux shaper in the image-forming optical unit, particularly in a parallel light-flux portion thereof, is in practical use today.
FIG. 14 schematically depicts a conventional optical scanning system having a surface-tilt correction function. This optical scanning system comprises a laser diode 101 for emitting an elliptically shaped light beam 102, a first image-forming optical unit 103, a polygon mirror 104 as a deflection means, and a second image-forming optical unit 105. The first and second image-forming optical units 103 and 105 are disposed upstream and downstream of the polygon mirror 104 with respect to the direction of travel of the laser beam towards a photosensitive member 106, respectively. Each of the first and second image-forming optical units 103 and 105 has different refractive forces in the primary and secondary scanning directions. The elliptically shaped light beam 102 from the laser diode 101 is focused in a predetermined size on the photosensitive member 106 by utilizing a two-step image forming function by the first and second image-forming optical units 103 and 105. The first image-forming optical unit 103 has a light-flux shaper 108 disposed in a parallel light-flux portion 107 of the first image-forming optical unit 103 to control the beam configuration so as to be focused on the photosensitive member 106.
The optical scanning system of FIG. 14 also comprises a plurality of surface-tilt correction lenses disposed on a light path defined by the first and second image-forming optical units 103 and 105. These lenses act to compensate for surface tilts of a plurality of reflecting surfaces of the polygon mirror 104, which may be caused by an undesirable oscillatory or wobbling motion of a rotary shaft of the polygon mirror 104 and/or inaccurate assemblage of the polygon mirror 104.
Japanese Laid-open Patent Publication (unexamined) No. 62-30214 discloses an optical scanning system wherein for the purpose of minimizing the required surface area in which various optical elements comprising the optical scanning system are accommodated, optics on the incident side of the polygon mirror and those on the reflection side of the polygon mirror are disposed one above the other without interfering with each other while an incident light path of the former forms an angle with respect to a reflected light path of the latter. Furthermore, the optical scanning system is so designed as to cause a light beam to be incident on the polygon mirror from the direction along the centerline of a field of scan in order to simplify non-uniformity of the scanning speed on a plane to be scanned, i.e., to make the scanning speed on such plane symmetric with respect to the centerline of the field of scan. Also, to reduce the overall size of the optical scanning system, the optics on the incident side of the polygon mirror includes a lens or lenses which refract the beam from a light source prior to the incidence thereof on the polygon mirror at the aforementioned angle.
Recently, there has been an increasing demand toward a highly accurate image formation with high density printing. To achieve this, the use of a second image-forming optical unit having different sectional configurations in the primary and secondary scanning directions is preferred.
Japanese Laid-open Patent Publication (unexamined) No. 58-93021 discloses a cylindrical lens employed in an optical scanning system. Although the formation of the cylindrical configuration is relatively easy, it is difficult to correct curvature of field in both the primary and secondary scanning directions and, hence, the use of the cylindrical lens imposes a limitation on the highly accurate image formation.
Japanese Laid-open Patent Publication (unexamined) Nos. 58-179813 and 58-179814 disclose an optical scanning system employing a troidal lens. The troidal surface is difficult to make because it requires forcible bending of the cylindrical surface, thus imposing a limitation on the highly accurate image formation.
Furthermore, because each of the lenses as disclosed in the Japanese publications referred to above has no f-.theta. characteristics, the system requires an electric circuit for performing correction.
FIGS. 15 and 16 show graphs indicating the curvature of field and an f-.theta. characteristic in one of the prior art references, respectively. In FIG. 15, a solid line indicates the curvature of field in the primary scanning direction, while a dotted line indicates the curvature of field in the secondary scanning direction.
An alternative to achieve the highly accurate image formation with high density printing is to arrange a light-flux shaper such as, for example, a masking member having a slit in one of image-forming optical units at a location not affected by refraction. By so doing, it is possible to control the convergent beam configuration on the plane to be scanned.
FIGS. 17A and 17B show a graph indicating the beam configuration in the primary scanning direction and the beam configuration in the secondary scanning direction in the said one of the prior art references, respectively.
Japanese Laid-open Patent Publication (unexamined) No. 56-141662 discloses an optical scanning system wherein a masking member having a slit is disposed in a parallel light-flux portion. Although the installation and positioning of the masking member are relatively easy, the parallel light-flux portion must be provided in one of image-forming optical units, making it difficult to ensure the simplicity and reliability of the system. Accordingly, there is also a limitation in obtaining a highly accurate convergent beam.
Japanese Laid-open Patent Publication (unexamined) No. 59-214012 discloses a laser printer wherein a masking member having a slit is disposed immediately before the plane to be scanned. This printer is at a disadvantage in that because the masking member must be placed remotest from a light source and, hence, the installation and positioning thereof are difficult. In addition, there is a limitation in reducing the size of the system.
Furthermore, in order for the system as disclosed in the Japanese Laid-open Patent Publication No. 62-30214 referred to above to satisfy the aforementioned incidence conditions by refracting the light beam, it is necessary to pass the light beam through a peripheral portion of the lens of a large diameter, resulting in an increase in the size of the optics on the incident side. Although it is possible to use such a lens by removing an unnecessary portion thereof processing thereof takes a lot of time, resulting in an increase in manufacturing cost.
The accuracy with which the light path is changed by the lenses in the optics on the incident side depends much upon the processing accuracy of the lenses themselves, and a decreased processing accuracy is likely to change the position of incidence on the polygon mirror, to thereby lower image-forming characteristics. Also, temperature changes cause a change in refractive index of the lens, thus changing the light path.